1. Field of the Invention
The present invention relates to the measurement of the density and viscosity of a liquid and more particularly to a densitometer which may be immersed in a liquid to measure these properties.
2. Description of the Prior Art
Laboratory instruments are available which measure liquid density with appreciable accuracy, involving the careful weighing of known volumes of liquid with precision laboratory balances. In such instruments, the density is obtained by calculating the ratio of the weight of the liquid to the volume. Accurate hydrometers containing weighted floats and graduated walls are placed in the liquids being measured and the volume of liquid displayed is carefully measured along the graduations. Here the accuracy is on the order of a percent. Dynamic devices have been built employing the momentum exchange between a moving liquid and balanced beams. The output of such devices is an analog signal with accuracies on the order of 1/4%. A second dynamic measurement is by means of a tuning fork with tines into which liquid whose density is being measured is pumped. The frequency of vibration of the tuning fork has a higher value when empty and a lower value when filled with liquid. The variation in frequency is a function, in part, of the density of the liquid. Using a calibration technique, the density of the contained liquid may be obtained to accuracies on the order of one part in 10.sup.3 or 10.sup.4. The tuning fork method is relatively slow and discontinuous, requiring access to the tuning fork, and an emptying step. Should one wish to make continuous measurements, simple immersion is ruled out since the normal mode of vibration of a tuning fork produces turbulence and an extremely strong damping effect. Accuracy in the measurement of the resonant frequency of a resonant system is strongly dependent on the Q of the resonent system. Since excessive turbulence reduces the Q of the oscillator, the accuracy of measurement is greatly reduced. To avoid inaccuracy, non-turbulent resonance is essential.